having problems with seires material

[sirwan]

Hi everybody
I have a model of 2D RCS frame composed of nonlinear RC columns and steel beams (fiber section) with 2D joint elements. The joint element implements a series material for the central node and the external nodes are rigidly connected to the column and beam. The series material uses a steel02 material and a hysteretic material to account for deformations of the joint. The problem emerges when I do eigen value analysis. The results are not the same if you put eigen value analysis before and after the gravity analysis! for the first mode in case doing eigen value analysis before gravity analysis you will get a period of 2.33 and placing it after gravity analysis the period would be about 1 sec. The period of 2.33 also occurs when the stiffness of series material is set to zero!, I mean the stiffness of series material sounds to not be considered in first case, unless you first do gravity analysis. here is the codes to construct series material. Any suggestions to solve this problem would be so appreciated!
"Best Regards"

#steel02 material parametrs
#story 1
set PSh_matTag 21
set PSh_Mps [expr 3031021936*$N*$mm]; #joint moment strength
set PSh_Kes [expr 1.638390236e12 *$N*$mm]; # joint stiffness
set PSh_b 0.02
set PSh_R0 18.5
set PSh_cR1 0.96
set PSh_cR2 0.15
uniaxialMaterial Steel02 $PSh_matTag $PSh_Mps $PSh_Kes $PSh_b $PSh_R0 $PSh_cR1 $PSh_cR2 0 1 0 1;

#Hystretic Material Parametrs
set H_matTag 25;
set s1p [expr 4564406925*$N*$mm];
set e1p 0.00108;
set s2p [expr 5071563250*$N*$mm]; #joint bearing strength
set e2p 0.05
set s3p [expr 329651613*$N*$mm];
set e3p 0.1
set s1n -$s1p
set e1n -0.00108
set s2n -$s2p
set e2n -0.05
set s3n -$s3p
set e3n -0.1
set pinchX 0.2
set pinchY 0.5
set damage1 0.015
set damage2 0.0
uniaxialMaterial Hysteretic $H_matTag $s1p $e1p $s2p $e2p $s3p $e3p $s1n $e1n $s2n $e2n $s3n $e3n $pinchX $pinchY $damage1 $damage2
set SeriesMat1 105;
uniaxialMaterial Series $SeriesMat1 $H_matTag $PSh_matTag